Model Systems in Neuroethology

Walking in the Cockroach

Animal

Cockroach, Periplaneta americana

"The cockroach is a primarily nocturnal insect of the order Blattaria.
The order contains a number of genera including several that are infamous
as domestic pests. The typical domestic cockroach is anywhere between a
quarter of an inch and one inch in length, and brown to black in color.
Their six-legged, oblong chitinous bodies are adorned with a pair of anteriorally
positioned feelers which are often as long as one-third of their overall
body length.

Cockroaches typically nest inside walls, ventilation ducts, heating devices,
baseboards and similar places. Their preferred nesting sites are in dark,
dusty areas free of vibration or noise. They tend not to be seen a great
deal during the day, preferring instead to leave their nests at night in
search of food and water. Food left outside of sealed plastic containers,
dripping taps, and other sources of moisture and organic debris will attract
hungry roaches. Conveniently enough, most bathrooms and kitchens provide
these in abundance.

Cockroaches can travel within a building by following plumbing lines
and eating their way through the very walls themselves, as well as by more
obvious means such as doors and windows. Cockroaches are accomplished climbers
and can move extremely quickly when threatened. They are hardy creatures,
able to survive on a variety of forms of food and with little water. Further,
they adapt quickly to poisons and reproduce rapidly. The typical gestation
period for a cockroach egg is on the order of a few weeks."

Behavior

The behavior being investigated is the simple act of walking, or the
rhythmic, sequenced, coordinated movement of the insects' limbs. The act
of walking for one limb can be defined as follows:

"The cyclic movement of a walking leg consists of two parts, the
power stroke (also stance phase or support phase) and the return stroke
(also swing phase or recovery phase). During the power stroke, the leg
is on the ground where it can support and propel the body. In a forward-walking
animal, this corresponds to a retraction movement of the leg. During the
return stroke, the leg is lifted off the ground and swung to the starting
position for the next power stroke." (Cruse 1990)

It obviously follows that the behavior is intrinsically cyclic. Furthermove,
behavioral evidence has shown that the motion of the 6 legs is coupled in
such a way as to produce coordinated walking. Obviously, there has to be
some neural circuitry responsible for this cyclic, coordinated behavior.
However, from the natural behavior one cannot determine whether the rhythmic
nature of the movements is generated by this circuitry or is the result
of sensory feedback.

A considerable amount of research has been done in an attempt to answer
this question, and the majority of the evidence favors the former over the
latter. In other words, the cyclic motor output is generated internally
within the neural circuitry (although sensory feedback is important for
modulating the actual frequency of motion). The neural circuits that generate
this rhythmic output are called Central Pattern Generators
and are described in some detail below.

Neural Substrates

The neural mechanisms underlying the rhythmic motor output were first
described by Pearson. (1970) Figure 1 below is a hypothetical diagram of
the circuitry that could produce the necessary output. Neurons labeled "5"
and "6" are levator motoneurones; "Ds" is a depressor
motoneuron. Bursting interneuron "b.i." phasically drives all
of these, with excitatory connections to "5" and "6"
and inhibitory connections to "Ds".

Figure 1

In later work, Pearson was able to locate the implicated neurons and
confirm connections between them. (1975) Figure 2 shows the actual locations
of two of the neurons (located in the metathoracic ganglion) described in
the figure 1.

Figure 2

Later work has shown that it is actually a collection of neurons rather
than a single interneuron (such as "b.i.") that is responsible
for generating and maintaining the rhythm. Such networks are known as Central
Pattern Generators, and they are currently thought to be responsible
for generating a variety of rhythmic behaviors in a variety of animals.
(Delcomyn 1980)

Line drawing of cockroach from Chapter 19 of Dave Stephen's online course notes for BIO 101
at the University of Nebraska. Figure 1 from Pearson et al. 1970; Figure
2 from Pearson et al. 1975

Selected References

Early classics:
Pearson, K. G., and C. R. Fourtner. (1975) Nonspiking Interneurons in the
Walking System of the Cockroach. Journal of Neurophysiology, 38
: 33-52.